Liquid-crystal display devices use a backlight unit that irradiates the liquid-crystal panel (LCD) with light from behind, thereby illuminating the liquid-crystal panel. The backlight unit is basically composed of an illuminating light source, a lightguide that diffuses the light emerging from the light source so as to irradiate the liquid-crystal panel, and a diffusion film that renders the light uniform as it is radiated from the lightguide.
Among those components of the backlight unit, the diffusion film is manufactured from a transparent support such as PET (polyethylene terephthalate) on which is formed a light diffusing layer comprising polymer particles having optical transparency. The diffusion film is provided on the side of the lightguide from which light issues and, as mentioned above, it is used to ensure that the light emerging from the lightguide is diffused uniformly, thus requiring good light diffusion and transmission.
JP 7-218705 A discloses a light diffusing film which, in order to strike a balance at increased level between light transmission and diffusion, comprises a substrate having a layer of a composition coated on one or both sides of it, the composition having 50-500 parts by weight of acrylic particles dispersed in 100 parts by weight of one or more light-transmitting resins in admixture, the acrylic particles having a refractive index different from that of the one or more light-transmitting resins in admixture by a value in the range of 0.01-0.15 and also having an average particle size of 1-50 μm. According to JP 7-218705 A, a light-diffusing film striking a balance between good light-transmitting and diffusing properties can be obtained that features a total light transmittance of at least 90% and a haze of at least 85%.
JP 11-194204 A discloses a light diffusing sheet which, in order to strike an improved balance between visual field enlarging capability and front brightness, has transparent light diffusing particles dispersed in a transparent matrix resin, the absolute value between the refractive index of the matrix resin and that of the light diffusing particles being in the range of 0.05-0.24, and the concentration of the particles in the sheet being provided with such a gradient that it gradually varies in the direction of thickness.
PATENT DOCUMENT 1: JP 7-218705 A
PATENT DOCUMENT 2: JP 11-194204 A